Image displays of mobile devices such as mobile phones, smart phones, and personal digital assistants (PDAs) have a touch panel mounted therein or a cover glass disposed thereon for surface protection. As such a touch panel or a cover glass, a chemically strengthened thin glass sheet with a thickness of 1.6 mm or less is commonly used. Chemical strengthening by alkali ion exchange is commonly used as an effective method for chemically strengthening thin glass sheets.
When a glass sheet produced by a float process is chemically strengthened, the glass sheet may be warped. It has been believed that this warping is caused by a tin layer that is formed in one surface (bottom surface) of the glass sheet kept in contact with molten tin in a float bath by entry of the tin component into the glass. In other words, it has been believed that occurrence of warping of a glass sheet after chemically strengthening is caused by a difference in compressive stress between the bottom surface and the top surface (i.e., the other surface of the glass sheet kept out of contact with the molten tin during formation of the glass into the sheet). This difference in compressive stress is produced by the following mechanism: during chemical strengthening by alkali ion exchange, the presence of the tin layer formed in the bottom surface reduces the amount of K+ ions entering through the bottom surface, and as a result, makes it smaller than the amount of K+ ions entering through the top surface.
In a chemical strengthening method for float glass proposed in Patent Literature 1, the top surface of the glass kept out of contact with molten tin is subjected to chemical treatment for reducing the concentration of sodium ions in the top surface prior to chemical strengthening. It is believed that this chemical treatment restricts alkali ion exchange in the top surface during the chemical strengthening, leads to a decrease in the difference between the amount of alkali ions entering through the top surface and the amount of alkali ions entering through the bottom surface, and as a result, reduces warping of the glass sheet after the chemical strengthening. As used herein, the chemical treatment is a treatment in which an oxidizing gas such as chlorofluorocarbon gas, hydrogen fluoride (HF) gas, or sulfur dioxide (SO2) gas is blown onto the surface of the glass sheet to allow the gas to react with the sodium component on the surface of the glass sheet.
On the other hand, in order to prevent damage to the surface of a glass sheet in the processes of producing, transporting, and processing the glass, it has been proposed to blow SO2 gas onto the surface of the glass sheet in the production process to allow the SO2 gas to react with an alkali component contained in the glass and thus to form a protective coating of sodium sulfate (salt cake) or the like on the glass surface (Patent Literature 2). Since the bottom surface is more susceptible to damage in the glass production process because it comes into contact with a conveyor roll, the protective coating needs to be sufficiently formed at least on the bottom surface.